Trace tree: let users turn aggregated cost on parent rows back on

[ralf-kinkel-comena]

#17590 (v4.39.0) removed the ∑ subtree cost from tree rows without an opt-out. For pipeline and agent traces, where spans group many generations, the cost per step is what we look at most. Now it only shows in the Timeline view, and the detail panel only sums for root observations.

Please keep your default but add a view option next to "Show Cost", e.g. "Show aggregated cost", that renders node.totalCost with ∑ as before. The value is still computed in treeBuilding.ts, so this is a small change.